The present invention relates to a mobile lifting crane that uses a boom hoist drum and rigging to change the angle of the boom. The invention provides a way to transport the boom hoist drum and rigging between job sites when the crane is disassembled for transport so that the boom hoist line does not have to be re-reeved when the crane is set up at the new job site.
Mobile lift cranes typically include a carbody having moveable ground engaging members; a rotating bed rotatably connected to the carbody such that the rotating bed can swing with respect to the ground engaging members; a boom pivotally mounted on a front portion of the rotating bed, with a load hoist line extending there from, and counterweight to help balance the crane when the crane lifts a load. There are different ways of changing the angle of the boom with respect to the rotating bed during crane operation, including using hydraulic cylinders mounted between the boom and the rotating bed. However, more commonly a boom hoist drum and rigging are used to change the boom angle. Many cranes also use a mast mounted on the rotating bed to support the rigging, including the boom hoist rigging, so that it may transfer the forces from lifting a load to the rear of the carbody and the counterweight. The boom hoist rigging must carry large tension loads, supporting not only the boom, but counteracting the angled force applied by the boom as it supports the load. The boom hoist rigging includes the boom hoist line extending from the boom hoist drum and reeved through a lower equalizer (which may be attached to the top of the mast), and an upper equalizer with multiple parts of line so that the large tension loads on the rigging are distributed over the multiple parts of line. The upper equalizer is usually connected to the boom top by fixed length pendants. The length of the pendants is chosen to correspond with the length of the boom.
Since the crane will be used in various locations, it needs to be designed so that it can be transported from one job site to the next. This usually requires that the crane be dismantled into components that are of a size and weight that they can be transported by truck within highway transportation limits. The ease with which the crane can be dismantled and set up has an impact on the total cost of using the crane. Thus, to the extent that fewer man-hours are needed to set up the crane, there is a direct advantage to the crane owner. If the boom hoist line can remain reeved through the lower and upper equalizers during the transportation operation, the line will not have to be re-reeved though the sheaves on the equalizers when the crane is set up again.
Several models of cranes have been designed to accomplish transporting the crane without unreeveing the boom hoist line. For example, U.S. Pat. No. 5,484,069 discloses a crane where the upper equalizer is disconnected from the boom top and connected to the mast (in this case the mast is part of a gantry) for transport. Since the gantry has the lower equalizer attached to it, and since the gantry does not need to be disconnected from the rotating bed, to which the boom hoist drum is attached, the boom hoist line does not need to be pulled out from the lower equalizer (mast top sheave assembly) and the upper equalizer during the disassembly process.
For some very large cranes, the size of the mast and the boom hoist drum are so large that they cannot both be transported on one truck. In that situation, the prior art practice has been to pull all of the boom hoist line onto the boom hoist drum so that it does not run through the sheaves on the mast top. The mast can then be disconnected from the rotating bed and transported separately. Unfortunately, this requires the boom hoist line to be reeved through the sheave assemblies in the lower and upper equalizers when the crane is set up again, a time consuming process. Thus it would be a great advantage if a system could be developed that allowed such very large cranes to be transported without having to pull the boom hoist line from the equalizers for transport.